Wind (eolian) erosion is usually mentioned in the scientific literature as wind
picking up sand particles (deflation) and “sandblasting” the bedrock
(corrasion).1,2 The most visible results are sand deposits (dunes
and associated forms) and strange “mushroom” rocks.

Little is however said or taught about the possibility of wind excavating large
hollows in massive rocks. The most obvious features that come to mind are tafoni
(singular tafone) which is defined as

“A hollow, produced by localized weathering on a steep face. Rock breakdown
typically takes place by granular disintegration or by flaking, and the hollow shows
a tendency to grow upwards and backwards.”3

Although most authors seem to emphasize localized weathering, mineral constituents
inside the host rocks and local fracture concentration as cause for their formation,
some have at least considered the role of wind in the overall excavation process.4

Wind “eats” rock

If winds were consistent for longer periods in the past, such an origin for many
of the arches might be possible.

I believe that wind plays a more significant role in the formation of tafoni. During
microclimate research in a salt mine near the city of Turda, Transylvania, Romania,
I witnessed the formation of many “megascallops”—large (up do
a meter long and 30 cm diameter at the wider end) scallop or spoon-like excavations—in
rocksalt resulting from the opening new air shafts. The cause was the sudden increase
in fresh air flow from the surface through the shaft. The fresh humid air being
unsaturated in salt aerosols, unlike the normal, near-stagnant mine atmosphere would
be able to dissolve the rocksalt in areas where turbulence ensured a longer air-rocksalt
contact. These were generally in the upper corners of the mining galleries and on
one location dozens of parallel megascallops formed in less than 10 years. There
were no significant chemical inhomogeneities in the rocksalt to explain such features
by selective dissolution. Microclimate measurements as well as smoke experiments
have confirmed the major role of air turbulence in creating these features.

The presence of tafoni on Mars5
further emphasizes the role of wind in “tafonisation”.

Water can too

Similar scalloping morphologies caused by air currents also occur in ice as exemplified
by the superbly scalloped walls of Arches Cave in the Khumbu Glacier (Mt. Everest,
Nepal; figure 1).6 They
also occur when wind blows over compacted snow.7
In these cases the mechanism is sublimation of homogenous crystalline matter.

Dissolutional scalloping and fluting is common in limestone, gypsum and salt caves.
Subcritical turbulent flow of unsaturated water has been proven to be the mechanism
responsible for scalloping in these situations.7 (Figure 2).

A common mechanism

Figure 2. Tafoni in Cretaceous sandstone at Stone Garden near Tumbler
Ridge, British Columbia, Canada. The vertical jointing does not affect/control tafonization.
There seems to be no case hardening nor is there any overhanging ‘brow’.

There are many morphological similarities between tafoni (figure 3) and these rocksalt
excavations and I suggest that air turbulence plays a significant role in the formation
of tafoni. The origin of tafoni is undoubtedly polygenic, with variable porosity
and matrix mineralogy acting as initiators of tafonisation. A more porous area in
sandstone for example will tend to accumulate more pore water and if freezing occurs,
cryoclasty will tend to cause granular disintegration. Once a tiny excavation forms,
air flow will become more turbulent and deflation will occur, with a tendency for
the excavation to deepen along turbulence (eddy) pathways; the larger the resulting
excavation, the more airborne particles become available for corrasion. Studies
have shown that tafoni tend to deepen rapidly as they expand (positive feedback)5.
If the wind is consistent and persistent, large tafoni can form in a matter of years.

Some authors5 have pointed out that case hardening occurs on the outer
roof areas of tafoni the result of persistent eddies that would further the specific
tafoni excavation.

From tafoni to arch

What would happen if tafoni formed in narrow inhomogeneous sandstone ridges (“rock
fins”)? If the polygenetic conditions are consistent and persistent enough,
they may very well perforate the ridge from one side to the other creating arches
like the ones in the Arches National Park in Utah. Once a perforation like that
occurred, the turbulent airflow would increase, as well as preferential, contour
cryoclasty as now the open rim will provide more porosity for water retention. Collapse
will significantly enlarge arches until certain, temporary equilibrium is reached.

Wall Arch was one of the largest natural arches in Arches National Park, Utah, before
it collapsed in August 2008.

Cave (karst) scalloping and pothole distribution reveals that these features almost
exclusively form on the walls and bedrock of conduits, never in the concavity of
sharp bends where water flow impact is frontal. The reason is that coherent and
persistant turbulence (vital for scalloping and potholing) occurs where parallel
flow is affected by rockwall rugosity. Similarly, one would expect that tafoni preferentially
form on the sides of narrow canyons, and statistically they do. In the Castle Rocks
State Park, Idaho, tafoni form mostly in the main cluster of spires, rather than
in isolated inselbergs.5

If winds were consistent for longer periods in the past, such an origin for many
of the arches might be possible. I acknowledge that I have not visited the site
and am relying entirely on photographic and video sources as well as field data
elsewhere. Very strong and persistent seasonal winds were present during the deglaciation
period in the Late Pleistocene even at mid-latitudes.8

When?

Oard9 attributes the arches
and natural bridges in Utah to Late Flood mechanisms. Natural bridges are different
from arches in that they span an existing or dry stream which is believed to have
undercut them. They are often excavated in relatively homogenous rocks unlike arches
which always have a harder rock forming the roof and a softer one below. According
to Oard, the arches would have formed during either the Sheet-flow or Channelize-flow
Phase of the Retreating Stage of the Flood, while natural bridges probably formed
during the Channelized Phase.10
Oard’s assumption is that somehow, as incised valleys formed during the Retreating
Phase, undercutting of less resistant rock under a more resistant layer. However,
this does not really answer the objection Oard himself quotes from the literature:

I think that these arches formed towards the end of the Ice Age; during rapid deglaciation.

“Arch formation cannot be due solely to weathering and erosion, however, because
these processes are not restricted to the sides of arches in rock fins. There must
be some factor that locally enhances the effects of erosion within a rather small
part of the rock fin to produce an arch. How erosion is localized within the rock
fin to form an arch is enigmatic.”11

I suspect that there is nothing enigmatic about such localization once persistent
seasonal air turbulence is inferred. Oard’s explanation is most likely valid
in the case of natural bridges and probably for the formation of rock fins during
rapid downcutting of incised valleys. In fact most rock fins represent meander necks,
narrow ridges of rock separating the two sides of a hairpin meander in an incised
valley. I think that these arches formed towards the end of the Ice Age; during
rapid deglaciation. Those were times when winds were very strong and persistent,
especially since most mid-latitude areas were completely deforested, allowing for
extensive eolian erosion as is current on Mars.12
In fact climate patterns were most likely very different from the present ones as
the thermohaline circulation system was massively disrupted by the sudden input
of huge amounts of fresh meltwater coming from the rapidly-melting ice sheets.13

Conclusion

The role of early post-Flood climate in geomorphology has not been investigated
to any extent by young-earth creation scientists, although it may in fact have played
a significant role in sculpting at least some of the landmarks of every continent.
Under such circumstances, any present attempt—including this one—cannot
but be speculative in nature. I believe that post-Flood paleoclimate reconstructions
are needed and they could be assisted if not complemented by geomorphic investigation.
Furthermore, such reconstructions may prove useful for future faunal and floral
distribution creationist studies, including anthropology.

Rodriguez-Navarro, C., Doehne, E. and Sebastian, E., Origins
of honeycomb weathering: the role of salts and wind, GSA Bulletin 111(8):1250–1255, August 1999; http://bulletin.geoscienceworld.org/cgi/content/abstract/111/8/1250.Return to text.

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Comments closed

A reader’s comment

Robert B.,Canada, 5 November 2010

I enjoy geomorphology and this article hits on points I care about, namely like processes create like results. Wind can do what water does for the same reasons.

Especially vortices in these forces is increasingly being seen as a great creative force. Not just wind/water moving but the vortices/turbulence within it. Interesting about the scallop case.